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# attach(device:completionHandler:)

Attaches a USB device to the controller.

```
func attach(device: any VZUSBDevice, completionHandler: @escaping ((any Error)?) -> Void)
```

## Parameters

`device`

The USB device to attach.

`completionHandler`

A block the framework calls after the device attaches, or on an error. The error parameter that passes to the block is `nil` if attaching is successful. The framework calls the block on a VM’s queue.

## Discussion

If the device successfully attaches to the controller, it appears in the [`usbDevices`](/documentation/Virtualization/VZUSBController/usbDevices) property, the framework sets its [`usbController`](/documentation/Virtualization/VZUSBDevice/usbController) property to point to the attached USB controller, and the completion handler returns `nil`.

If the device has a previous attachment to the current USB controller, or to another USB controller, the attach function fails with [`VZError.Code.deviceAlreadyAttached`](/documentation/Virtualization/VZError/Code/deviceAlreadyAttached). If the controller can’t initialize the device correctly, the attach function fails with [`VZError.Code.deviceInitializationFailure`](/documentation/Virtualization/VZError/Code/deviceInitializationFailure).

You need to call this method on the virtual machine’s queue.

## See Also

[`VZUSBDevice`](/documentation/Virtualization/VZUSBDevice)

A protocol that represents a USB device in a VM.


---

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